When looking at the Message Audit Log at emails from gmail.com email addresses, you notice a lot of abort message entries.
As evidenced by recent packet captures for customer seeing this issue, Google tries to send a large email despite the size specification being provided in the ESMTP response. Here is an example:
220 smtp01.x.com ESMTP Symantec Messaging Gateway
250-smtp01.x.com says EHLO to 209.x.x.x:34896
MAIL FROM:<[email protected]> SIZE=24814870
552 5.3.4 message size limit exceeded at MAIL FROM
RCPT TO:<[email protected]>
503 5.5.1 RCPT TO without MAIL FROM
503 5.5.1 DATA without MAIL FROM
221 2.3.0 smtp01.x.com closing connection
The above communication showed as an abort message.
This issue is outside the Messaging Gateway, Google's MTA is not correctly reading the SIZE parameter.